Wet strength agent and method for production thereof

ABSTRACT

A method for preparing a wet strength agent comprising a first step of reacting a nitrogen-containing polymer with a hydrophobic compound to form hydrophobic side-chain substituents on the polymer, a second step of reacting the hydrophobised nitrogen-containing polymer obtained with a crosslinker to form a cationic nitrogen-containing resin, and a third step comprising forming of particles by emulsion polymerisation of one or more ethylenically unsaturated monomers in the presence of the wet strength resin formed. The invention further relates to a wet strength agent and resin. It further relates to the use of said agent and resin in cellulosic suspensions, the production of paper, preferably tissue paper, and paper, preferably tissue paper comprising a wet strength resin or agent.

[0001] The invention relates to a paper wet strength agent and a methodfor the production of such an agent. The invention further relates tothe production of paper comprising addition of the paper wet strengthagent to an aqueous cellulosic suspension and to paper comprising apaper wet strength agent. In addition, the invention relates to the useof a paper wet strength agent as an additive to an aqueous cellulosicsuspension.

BACKGROUND

[0002] In the papermaking art, wet strength agents likeepichlorohydrin-based resins, for example polyaminoamide epichlorohydrinresins have been used for a long time to enhance the strength of paper.Such resins are disclosed in U.S. Pat. No. 3,700,623 and U.S. Pat. No.3,772,076. The wet strength of a paper relates to its ability tomaintain physical integrity and to resist tearing, bursting, andshredding under use, especially under wet conditions. A furtherimportant property of wet strengthened paper is the softness, especiallyfor tissue paper or the like. The softness can be described as thetactile sensation perceived when holding or rubbing a paper across theskin.

[0003] U.S. Pat. No. 5,200,036 discloses a wet strength agent whichprovides paper with enhanced wet strength. A cationic polyaminoamideepichlorohydrin resin is modified by introduction of a polymerisableunsaturated hydrocarbon moiety thus providing it with ethylenicallyunsaturated side-chain substituents. The resin is then added tolatex-forming monomers whereby co-polymerisation occurs forming bondsbetween unsaturated polymerisable hydrocarbon moieties of the resin andthe latex-forming monomers. The reaction may be assisted by addition ofan emulsifier to obtain a desirable suspension of the formed latexparticles. Resins of the above-mentioned types are also used asemulsifiers. Usually, the resins are not effective enough when used as asole emulsifier and these are thus used in combination with anadditional compound.

[0004] U.S. Pat. No. 5,314,721 discloses a process for preparation ofvinyl polymer dispersions comprising resin based on a cationicpolyaminoamide whose terminal groups have been substituted withlong-chain aliphatic hydrocarbon radicals which have at least 7 carbonatoms and are derived from monocarboxylic acids. The product obtained isused as a sizing agent.

[0005] U.S. Pat. No. 4,416,729 discloses a method for preparing wetstrength additives comprising the steps of contacting a linearpolyamidoamine with an α,β-ethylenically unsaturated carboxylic compoundto form a substituted polyamidoamine, contacting the substitutedpolyamidoamine with a polyamine to form a branched polyamidoaminebearing a pendant amine moiety, and contacting the branchedpolyamidoamine with an epihalohydrin to form pendant curable ammoniummoieties on the branched polyamidoamine. U.S. Pat. No. 4,416,729 doesnot disclose use of the prepared wet strength additives for productionof tissue paper.

[0006] Although the above epichlorohydrin-based resins in someapplications show adequate wet strength and emulsifying properties, itwould be desirable to be able to provide further and improved wetstrength agents for paper and methods for providing such agents. Itwould also be desirable to be able to provide wet strength resins andagents exhibiting improved softness properties. Further, it would bedesirable to be able to provide further resins having improvedemulsifying properties.

THE INVENTION

[0007] According to the present invention, it has been found thatfurther and improved wet strength agents for paper can be obtained by acomposition containing polymeric particles and hydrophobic hydrocarbongroups providing side-chain substituents on wet strength resins. It hasalso been found a new method for the production of such wet strengthresins and agents. It has further been discovered that the wet strengthagents and resins produced by the method according to the presentinvention give paper improved softness properties without negativelyaffecting the absorbency properties.

[0008] More specifically, the invention relates to paper wet strengthagents comprising polymeric particles and wet strength resins comprisinga cationic nitrogen-containing polymer having hydrophobic side-chainsubstituents. The invention further relates to a method for theproduction of a paper wet strength agent comprising a first step ofreacting a nitrogen-containing polymer with a hydrophobic compound toprovide a nitrogen-containing polymer with hydrophobic side-chainsubstituents, a second step of reacting the product obtained with acrosslinker to form a cationic wet strength resin, and a third stepcomprising emulsion polymerisation of one or more ethylenicallyunsaturated monomers in the presence of the wet strength resin formed.Further, the invention relates to a paper wet strength agent obtainablefrom the method above. The invention further relates to a new wetstrength resin and a method for preparing a wet strength resin accordingto the two first steps as described herein. The invention also relatesto the production of paper comprising addition of a paper wet strengthresin or agent to a cellulosic suspension and to the use of a paper wetstrength resin or agent for the production of paper. The invention alsorelates to paper comprising paper wet strength resins and agents. Theinvention is further defined in the appended claims.

[0009] The present invention provides resins and agents having theability to impart improved wet strength properties to paper. Theinvention further provides a simple, convenient and effective syntheticroute for the preparation of wet strength resins and agents. Thereby,the wet strength resins and agents of this invention can be prepared inhigh yield.

[0010] The present invention also provides wet strength resins andagents which make it possible to produce paper having enhanced softnessproperties. The softness of a paper sheet can be estimated by means ofthe relative wet strength value, which is defined as the ratio betweenthe wet tensile index and the dry tensile index according to the formulaRWS (in %)=(WS/DS) 100 , where RWS stands for the relative wet strength,WS is the wet tensile index and DS is the dry tensile index of a paper.RWS is hence a measure of the softness of a paper; the higher the RWS,the higher the softness of the paper. The present wet strength resinsand agents also provide improved emulsifying properties and can be usedas sole emulsifiers without additional compounds which may give rise toundesirable foam formation.

[0011] The term “wet strength agent”, as used herein, refers to an agentcapable of imparting better wet strength properties to paper compared topaper containing no such agent. The wet strength agent comprises a wetstrength resin. The term “wet strength resin”, as used herein, refers toa resin capable of imparting better wet strength properties to papercompared to paper containing no such resin.

[0012] The method for the production of a paper wet strength agentcomprises a first step of reacting a nitrogen-containing polymer with ahydrophobic compound to provide a nitrogen-containing polymer withhydrophobic side-chain substituents, a second step of reacting theproduct obtained with a crosslinker to form a wet strength resin, and athird step comprising forming of particles by emulsion polymerisation ofone or more ethylenically unsaturated monomers in the presence of thewet strength resin formed. According to a preferred embodiment, nopolyamine having at least 2 secondary and/or primary amine moieties,added between the first and the second step, or after the second step,is reacted.

[0013] Suitably, the nitrogen-containing polymer is a polyaminoamide, apolyamine or other nitrogen-containing polymer. Preferably, apolyaminoamide is used which may constitute the reaction product of apolycarboxylic acid, suitably a dicarboxylic acid, and a polyamine. Bythe term “carboxylic acid” is meant to include carboxylic derivativessuch as anhydrides and esters. Suitable polycarboxylic acids includesaturated or unsaturated aliphatic or aromatic dicarboxylic acids.Preferably, the polycarboxylic acids contain less than 10 carbon atoms.Suitable polycarboxylic acids include oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acidand derivatives thereof. Mixtures of these compounds can also beapplied. Suitable polyamines include polyalkylene polyamines, e.gdiethylenetriamine, triethylenetetramine, tetraethylenepentamine,dipropylenetriamine and the like or mixtures thereof. Further, anypolyaminoamide prepared according to a method disclosed in EP 802 215A1, hereby incorporated by reference, may be used. Suitably, themolecular weight of the nitrogen-containing compound ranges from 100 to50000, preferably 500 to 10000. Suitably, the polyamine topolycarboxylic acid ratio is 0.49:1 to 1.49:1, preferably less than1.3:1, e.g. 1.3:1 to 0.7:1. Suitably, diethylenetriamine and adipic acidare reacted to form a polyaminoamide.

[0014] Suitably, the hydrophobic compounds used can contain groups ofcarboxylates or derivatives thereof. The hydrophobation reaction betweenthe nitrogen-containing polymer and the hydrophobic compound can beperformed via alkylation, vinylog addition or other reaction. Thevinylog addition may be illustrated by the following schematic reaction:

[0015] wherein WV-NH-WV represents a section of the nitrogen-containingpolymer, C═C—COOR represents a hydrophobic compound containing a vinylgroup. The vinyl group, i.e. the C═C group, of the hydrophobic compoundcan react with the nitrogen atoms of the polymer. R stands for ahydrophobic group of the hydrophobic compound which may be an alkyl,alkenyl, aryl, cycloalkyl or cycloalkenyl group. In case the vinylogreaction is applicable, the unsaturation of the vinyl group of thehydrophobic compound is turned saturated after having reacted with anitrogen atom of the polymer.

[0016] According to one preferred embodiment, the hydrophobic compoundis a saturated compound, or an unsaturated compound, resulting in anitrogen-containing polymer having saturated side-chain substituents.

[0017] The hydrophobic compounds can contain a hydrophobic groupcontaining up to 40 carbons, preferably 6-40 carbons, and mostpreferably 8-40 carbons.

[0018] The hydrophobic chains of the hydrophobic compounds can beattached to the nitrogen-containing polymer via a chain of atoms, whichcan contain at least one hetero atom, via a covalent bond.

[0019] The hydrophobic compound may be selected from (meth)acrylates,alkenyl(meth)acrylate, alkyl(meth)acrylamides, esters, ethers, diazocompounds, carboxylic acids, acid anhydrides epoxides, alkylsulphonates, alkyl sulphates and mixtures or derivatives thereofcontaining a hydrophobic group, preferably from alkyl(meth)acrylates,alkyl(meth)acrylamides, alkyl sulphonates, alkyl sulphates, diazocompounds, ethers, or epoxides or mixtures thereof, and most preferablyfrom alkyl(meth)acrylates, alkyl(meth)acrylamides or mixtures thereof.Examples suitably include α,β-unsaturated esters or amides like laurylacrylate, 2-ethylhexyl acrylate, dodecyl acrylate,N-alkyl(metha)acrylamides, N-alkylaminoalkyl(meth)acrylamides,N,N-dialkylaminoalkyl(meth)acrylamides,N-alkylaminoalkyl(meth)acrylates, N,N-dialkylaminoalkyl(meth)acrylates,hexyl chloride, 2-ethylhexyl chloride, octyl chloride, decyl chloride,dodecyl chloride, hexadecyl chloride, octadecyl chloride, ethyl epoxide,propyl epoxide, (n-, t-, I-) butyl epoxides, pentyl epoxide, hexylepoxide, 2-ethyl-hexyl epoxide, octyl epoxide, decyl epoxide, dodecylepoxide, hexadecyl epoxide, octadecyl epoxide, hexene, 2-ethyl-hexyene,octene, decene, dodecene, hexadecene, and octadecene.

[0020] The reaction is suitably carried out in water, neat or in othersolvent, e.g. in an organic solvent, e.g. methanol, ethanol, ethyleneglycol or the like, capable of at least partly dissolving the reactantswithout taking part in the reaction under the reaction conditions.Mixture of such solvents can also be used. The reaction is preferablycarried out in water. The molar ratio nitrogen-containing polymer (basedon amino mols) to hydrophobic compound can be at least 1:1, suitably 2:1to 99:1, preferably 3:1 to 40:1. The reaction temperature may range fromabout 25° C. to about 150° C., preferably from about 60 to about 90° C.

[0021] In a second step, the hydrophobised nitrogen-containing polymersare reacted with a crosslinker. The term crosslinker or crosslinkingagent, as used herein, is meant to denote a compound having the abilityto crosslink the resin and/or to form bonds to cellulosic fibres.Suitably, the crosslinkers, sometimes referred to as intralinkers in EP802 215 A1, describing various intralinkers, hereby incorporated byreference, can comprise epihalohydrins e.g. epichlorohydrin; diepoxides,diacrylates, dimethacrylates, diacrylamides, and dimethacrylamides andmixtures or derivatives thereof are used. Preferably, epichlorohydrin isused as crosslinker.

[0022] The reaction is suitably carried out in an aqueous solution, neator by use of other solvent than water, e.g. ethanol, propanol or thelike or mixtures thereof. Suitably, the solvent solvent can not reactwith the reactants under the reaction conditions used. Preferably, thereaction is carried out in water. The reaction temperature may rangefrom about 0° C. to about 150° C., preferably between from about 4 toabout 80° C. The molar ratio of the hydrophobised nitrogen-containingpolymer (based on amino-mols) to crosslinker in the reactant compositionmay be 10:1 to 1:10, preferably 2:1 to 1:2.

[0023] In a third step according to the invention, the method comprisesemulsion polymerisation of one or more ethylenically unsaturatedmonomers in the presence of the wet strength resin as formed after thesecond step herein. The monomers may be selected from styrene,butadiene, vinyl acetate, vinyl amide, alkyl(meth)acrylamide,alkyl(meth)acrylate, e.g. methyl (meth)acrylate, butyl(meth)acrylate,butyl, glycidyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,dodecyl(meth)acrylate, octadecyl(meth)acrylate; (meth)acrylonitrile,isoprene, or 1,6-hexandiol diacrylate, or mixtures or derivativesthereof. As a result of the polymerisation process, the formed wetstrength resin can be anchored to the polymeric particles formedyielding a wet strength agent. As initiator of the polymerisationreaction, any conventional initiator can be used. For example, Wako VA044 can be used. Preferably, the initiator is water soluble. In theemulsion polymerisation reaction, the wet strength resin works as anemulsifier during the particle formation. The formed particle may becomposed of one sole or a mixture of unsaturated ethylenicallypolymerisable monomers as above exemplified. The reaction is preferablycarried out in water, organic solvents, e.g. ethanol, propanol or thelike, or mixtures of organic solvents or mixtures of both water andorganic solvents. The reaction temperature may range from 4° C. to about150° C., preferably from about 30 to about 90° C. The weight ratio resinto monomer can be 100:1 to 1:100, suitably 10:1 to 1:50.

[0024] The invention further relates to a method for preparing a wetstrength resin comprising the first and second steps of the method asabove described.

[0025] The invention also relates to a wet strength agent comprisingpolymeric particles and a wet strength resin comprising a cationicnitrogen-containing polymer having saturated hydrophobic side-chainsubstituents and a derivative of a crosslinker.

[0026] The polymeric particles can be formed from polymerised monomersas described above. Preferably, monomers are selected from styrene,acrylates and mixtures or derivatives thereof.

[0027] The cationic nitrogen-containing polymer has saturatedhydrophobic side-chain substituents and derivatives of a crosslinkerattached to the nitrogen atoms of the polymer.

[0028] Examples of suitable nitrogen-containing polymers includewell-known available commercial products which may be prepared asdescribed above or according to conventional methods known in the art.Examples of suitable nitrogen-containing polymers includepolyaminoamides, alkyl polyamines, polyimines, and polyvinylamines.

[0029] Hydrophobic saturated side-chain substituents are attached to thenitrogen atoms of the nitrogen-containing polymer. The term hydrophobicside-chain substituent is here meant to include hydrophobic groupscontaining e.g. hydrophobic linear or branched hydrocarbon chains whichcan be linked, e.g. via a hetero atom by a covalent bond, to a nitrogenatom of the nitrogen-containing polymer. Hydrophobic groups may alsoinclude cyclic chains including cyclic hydrocarbons. Combinations oflinear, branched and cyclic hydrocarbons are also included in theconcept of hydrophobic groups.

[0030] The hydrophobic group of the hydrophobic side-chain can containup to 40 carbon atoms, preferably 6-40 carbon atoms, and most preferably8-40 carbon atoms.

[0031] The hydrophobic side-chain substituents may derive from e.g.alkyl(meth)acrylates, alkyl(meth)acrylamides, esters, ethers, diazocompounds, carboxylic acids, acid anhydrides, epoxides, alkylsulphonates, or alkyl sulphates, or mixtures thereof containing ahydrophobic group, preferably from alkyl(meth)acrylates,alkyl(meth)acrylamides, alkyl sulphonates, alkyl sulphates, diazocompounds, ethers, or epoxides or mixtures thereof, and most preferablyfrom alkyl(meth)acrylates, alkyl(meth)acrylamides or mixtures thereof.

[0032] Specific examples include substituents derived fromα,β-unsaturated esters or amides like lauryl acrylate, 2-ethylhexylacrylate, dodecyl acrylate, N-alkyl(metha)acrylamides,N-alkylaminoalkyl(meth)acrylamides,N,N-dialkylaminoalkyl(meth)acrylamides,N-alkylaminoalkyl(meth)acrylates, N,N-dialkylaminoalkyl(meth)acrylates,alkyl sulphonate, alkyl sulphates, hexyl chloride, 2-ethylhexylchloride, octyl chloride, decyl chloride, dodecyl chloride, hexadecylchloride, octadecyl chloride, ethyl epoxide, propyl epoxide, (n-, t-,I-) butyl epoxides, pentyl epoxide, hexyl epoxide, 2-ethylhexyl epoxide,octyl epoxide, decyl epoxide, dodecyl epoxide, hexadecyl epoxide,octadecyl epoxide, hexene, 2-ethyl-hexylene, octene, decene, dodecene,hexadecene, and octadecene.

[0033] Other suitable substituents may derive from substituted succinicanhydrides containing a group selected from alkyl, alkenyl, aralkyl, oraralkenyl, and ketene dimers or multimers. Further examples of suitablesubstituents may be derived from the compounds disclosed in WO98/39376,hereby incorporated by reference.

[0034] A derivative of a crosslinker can be attached to thenitrogen-containing polymer which makes it possible to create bonds tonitrogen-containing polymers and/or cellulosic fibres. Derivatives of acrosslinker can be derived from epihalohydrins e.g. epichlorohydrin,diepoxides, diacrylates, dimethacrylates, diacrylamides, anddimethacrylamides or mixtures or derivatives thereof may be used.Preferably, the crosslinker is derived from epichlorohydrin.

[0035] According to one preferred embodiment, the cationicnitrogen-containing polymer is either a polyaminoamide-epichlorhydrinresin or a polyamine-epichlorohydrin resin having saturated hydrophobicside-chains. Suitably, at least 10% and preferably up to about 100% ofthe nitrogen atoms of the cationic resin comprise cationic groups.Suitably, up to 100% of the nitrogen atoms of the resin comprisehydrophobic groups, preferably up to 50%, most preferably 5-30%.Suitably, the wet strength agent comprises a composition of polymericparticles and a wet strength resin dissolved in a solvent, preferablythe wet strength agent comprise an aqueous composition. Suitably, theaqueous composition has a solid content of 5-50 weight percent.

[0036] The invention further relates to a wet strength resin as abovedescribed.

[0037] The invention also relates to the use of the paper wet strengthresin and agent, as described above for the production of paper,preferably tissue paper. The use comprises addition of the resin oragent to an aqueous suspension containing cellulosic fibres. The amountof resin added to dry cellulosic fibres may be in any proportions,suitably 1-70, preferably 5-50, more preferably 15-50, and mostpreferably 25-50 kg/tonne dry cellulosic fibres. The grammage of theproduced paper suitably is lower than about 70 g/m², preferably lowerthan about 60 g/m², and most preferably lower than 40 g/m². The paperwet strength resin and agent are preferably produced as aqueousdispersions which comprise the resin, water and optionally emulsifiedparticles. The dispersion can then be added to an aqueous cellulosicsuspension to treat paper-forming cellulosic fibres. The paper wetstrength resin and agent may also be added to the produced paper andthus providing surface treatment of the paper, Further, the addition ofthe wet strength resin or agent may be added together with any otherchemical known in the art conventionally used in the production ofpaper, e.g. sizing agents, softeners, retention aids, dewatering agents,dry strength agents, charge control agents or any other conventionalchemicals, e.g. guars, carboxymethyl cellulose, polyacrylamide,polystyrene. Further, conventional fillers can be added thereto, e.g.clay, calcium carbonate, titanium dioxide, talc, aluminum silicate,calcium sulphate, calcium silicate or others described in WO 97/37080.Further, the wet strength agent may be added to the cellulosicfibre-containing suspension in any proportion. Before the wet strengthresin or agent are added to an aqueous cellulosic suspension, theaqueous dispersion containing the resin or agent may be subjected toremoval of toxic by-products by means of ion exchange, electrodialysis,enzymatical treatment, filtration, steam stripping or the like in ordernot to add any toxic products, e.g. chloropropandiol, dichloropropanolto the cellulosic suspension These methods are further described in forexample EP 666 242 A1, EP 510 987 A1 and WO 92/22601.

[0038] The invention further relates to a process for the production ofpaper, preferably tissue paper, comprising addition of a paper wetstrength resin and/or an agent as described and exemplified herein to anaqueous cellulosic suspension. The invention also relates to paper,preferably tissue paper, comprising a wet strength resin and/or an agentas described and exemplified herein. By tissue paper is generally meantitems such as facial, hand, and toilet tissues used as a personal careproduct which comprises two key elements: a substrate formed of a planarmaterial commonly known as tissue paper and an emollient which iscarried by the substrate. In this context, tissue paper also comprisesapplications for domestic and industrial use, such as wiping of objectsby means of kitchen rolls or the like. Tissue paper is generallyproduced from an aqueous suspension of cellulosic fibres, to whichsuspension wet strength agents have been added. The cellulosefibre-containing aqueous suspension is thereafter dewatered, suitably toa consistency of between about 7% and 25% water, suitably by means ofvacuum dewatering and pressing operations such as opposing mechanicalmembers, e.g. cylindrical rolls, to obtain a wet cellulosefibre-containing web. The dewatered web is further pressed duringtransfer and dried suitably by a stream drum apparatus known in the artas a Yankee dryer. Vacuum may also be applied to the web as well asmultiple Yankee dryer drums, whereby additional pressing is optionallyincurred between the drums, thereby forming tissue paper structures. Thesubstrate can either consist of a single ply of tissue paper or it cancomprise a laminate of two or more plies of tissue paper. In eitherevent, since the substrate is formed of tissue paper, it is contemplatedthat it will be relatively thin in comparison to its dimensions in itsmajor plane. As a relatively thin planar material, the substrate willhave two major surfaces. Four important physical attributes of tissuepapers are their strength, their softness, their absorbency,particularly for aqueous systems, and their lint resistance,particularly their lint resistance when wet, as further described inWO95/01478. Production methods for producing tissue paper are furtherdescribed in WO95/01478, hereby incorporated by reference. More specificapplications or uses of tissue paper include receiving and containingdischarges from the human body, which can be used to wipe portions ofthe human body to remove substances therefrom, and which can be used todeposit materials thereon. The inventional paper wet strength resin oragent suitably has hydrophobic side-chains containing 6-40 carbon atoms,preferably 8-40 carbon atoms. Hydrophobic side-chains may be derivedfrom (meth)acrylates, alkenyl(meth)acrylate, alkyl(meth)acrylamides,esters, ethers, diazo compounds, carboxylic acids, acid anhydrides,epoxides, alkyl sulphonates, alkyl sulphates and mixtures or derivativesthereof containing a hydrophobic group, preferably fromalkyl(meth)acrylates, alkyl(meth)acrylamides, alkyl sulphonates, alkylsulphates, diazo compounds, ethers, or epoxides or mixtures thereof, andmost preferably from alkyl(meth)acrylates, alkyl(meth)acrylamides ormixtures thereof. Other suitable hydrophobic side-chains may be derivedfrom substituted succinic anhydrides containing a group selected fromalkyl, alkenyl, aralkyl, or aralkenyl, and ketene dimers or multimers.Further examples of suitable hydrophobic side-chains may be derived fromthe hydrophobic compounds disclosed in e.g. WO98/39376, U.S. Pat. No.9,922,243, hereby incorporated by reference. The grammage of theproduced tissue paper suitably is lower than about 70 g/m², preferablylower than about 60 g/m², and most preferably lower than 40 g/m². Theamount of resin or agent added to a certain amount of dry cellulosicfibres may be in any proportions, suitably from about 1 to about 70kg/tonne dry cellulosic fibres, preferably from about 5 to about 50,more preferably from about 15 to about 50, and most preferably fromabout 25 to about 50 kg/tonne dry cellulosic fibres. According to onepreferred embodiment, a further dry strength agent is added incombination with the inventional paper wet strength resin or agent, e.g.starch, guar, carboxymethylcellulose (CMC) or a synthetic dry strengthagent such as anionic or amphoteric polyacrylamides, even though theaddition level of the inventional paper wet strength resin or agent tothe aqueous cellulosic suspension is from about 5 to about 50 kg/tonnedry cellulosic fibres. In order to adjust a suitable dry strength of theproduced tissue paper, a person skilled in the art can select a suitablehydrophobic wet strength resin or agent to obtain a desirable tissuepaper, whereas the wet strength of the tissue paper can be controlled byadding an appropriate amount of resin or agent to the aqueoussuspension. A tissue paper having a high relative wet strength canthereby easily be achieved.

[0039] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the gist and scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the claims. While theexamples herebelow provide more specific details of the reactions, thefollowing general principles may here be disclosed. The followingexamples will further illustrate how the described invention may beperformed without limiting the scope of it.

EXAMPLE 1

[0040] Reaction of a polyaminoamide (hereinafter also called PAIM)(produced from adipic acid and diethylene triamine) with a hydrophobiccompound (vinylog addition): 240 g (0.60 amino-mol equivalents) PAIM(53% solution in water) and 27.3 g (0.15 mol) 2-ethylhexyl acrylate(2-EHAc): were heated for 6 h and 30 min at 80° C. Subsequently, 176 gof water was added and the solution was cooled down to room temperature.Conversion of acrylate was: 99.7%.

[0041] 307 g of the above hydrophobised PAIM solution was reacted with30 ml epichlorohydrine (ECH) at 6° C. for 6 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased until 50° C. and a viscosity of 120 mPa s was reachedwhereupon 155 ml of water was added and the temperature was adjusted to65° C. to let the viscosity reach 120 mPa s. The reaction was finalisedby adding 11 ml of sulfuric acid (50%) adjusting the pH to 3.5.

[0042] Emulsion polymerisation: The ratio resin to styrene was 1:2. Asolution of 47 g of the above produced wet strength resin, 104 g waterand 1.5 ml defoamer (10% solution in water) was purged with nitrogen.The temperature was then increased to 50° C. whereupon 0.5 g Wako VA 044and 1 ml styrene were added to the solution. 10 min later, additionalstyrene was added (total amount: 25 g). After 5 h at 50° C., thetemperature was increased to 70° C. at which temperature the solutionwas kept for an hour.

EXAMPLE 2

[0043] Reaction of Polyaminoamide (PAIM) with a 2-ethylhexylacrylate(2-EHAc) (vinylog addition): 82 g (0.20 amino-mol equivalent) PAIM (52%solution in water), 1.84 g (0.01 mol) 2-ethylhexyl acrylate (2-EHAc) and43 g of water were heated for 2 h at 80° C. Conversion of acrylate:98.9%.

[0044] 15.4 ml epichlorohydrine (ECH) was added to 125 g of the abovehydrophobised PAIM solution at 6° C. for 6 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased to 65° C. and a viscosity of 120 mPa s was reachedwhereupon 86 ml of water was added. The temperature was raised to 65° C.and kept at 65° C. until the viscosity reached 120 mPa s. The reactionwas finalised by addition of 11 ml sulfuric acid (50%) adjusting the pHto 3.5.

[0045] Emulsion polymerisation:. The resin/styrene ratio was 1:0.5. Asolution of 88.5 g of the above wet strength resin, 92 g water and 1.5ml defoamer (10% solution in water) was purged with nitrogen. Thetemperature was increased to 45° C. 0.04 g Wako VA 044 and 2 ml styrenewere added whereafter the temperature was raised to 50° C. After 10minutes, additional styrene was added (total amount: 12 g). After 3 h at50° C., the reaction mixture was cooled down to room temperature.

EXAMPLE 3

[0046] 260 g (0.65 amino-mol equivalent) PAIM (53% solution in water)(Polyaminoamide, PAIM) and 25% 41.0 g (0.16 mol) dodecyl acrylate(vinylog addition) were heated for 4 h 30 min at 80° C. Subsequently,211 g water was added whereafter the mixture was cooled down to roomtemperature.

[0047] 302 g of the above hydrophobised PAIM was then reacted with 30 ml(0.20 mol) epichlorohydrine (ECH) at 6° C. for 4 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased until 50° C. and a viscosity of 120 mPa s was reached.185 ml water was then added and the temperature was raised to 65° C. andkept at that temperature until the viscosity reached 120 mPa s. Thereaction was finalised by addition of 10 ml sulfuric acid (50%)adjusting the pH to 3.5.

[0048] Emulsion polymerisation:. The resin/styrene ratio was 1:1. Asolution of 75.0 g of the above wet strength resin, 100 ml water and 1ml defoamer (10% solution in water) was purged with nitrogen. Thetemperature was increased to 50° C. whereupon 30 mg Wako VA 044 and 1 mlstyrene were added. After 10 minutes, additional styrene was added(total amount: 20.5 g). After 5 h at 50° C. the temperature wasincreased to 70° C. and set at that temperature for one hour.

EXAMPLE 4

[0049] In the emulsion polymerisation, butyl acrylate was used insteadof styrene. A solution of 75.0 g of the wet strength resin of example 3(13% solids), and 1.5 g defoamer (10% solution in water) was purged withnitrogen. The temperature was increased to 45° C. 0.03 g Wako VA 044 and2 ml butyl acrylate were then added whereupon the temperature wasincreased to, 50° C. After ten minutes, styrene was added (total amount:14.2 ml). After 2 h 50 min at 50° C., the temperature was increased to70° C. which temperature was kept for one hour.

EXAMPLE 5

[0050] 25% 2-ethylhexyl acrylate was used to hydrophobise PAIM. Emulsionpolymerisation:. A solution of 121 g of the wet strength resin ofexample 1 (solids 28%), 131 g water and 1 ml defoamer (10% solution inwater) was purged with nitrogen. The temperature was increased to 45° C.0.04 g Wako VA 044 and 2 ml of a monomer mixture (styrene: 1.6-hexandioldiacrylate=0.375:0.125) were added whereupon the temperature was raisedto 50° C. in 10 min. Subsequently, the monomer mixture was added (totalamount: 17 g). After 3 h at 50° C. the reaction mixture was cooled downto room temperature.

EXAMPLE 6

[0051] 25% 2-ethylhexyl acrylate was used to hydrophobise PAIM. Amonomer mixture of styrene with t-butyl acrylate (0.45:0.05) was used.Emulsion polymerisation: A solution of 121 g of the wet strength resinof example 1 (solids 28%), 131 g of water and 1 ml defoamer (10%solution in water) was purged with nitrogen. The temperature wasincreased to 45° C. 0.04 g Wako VA 044 and 2 ml of a monomer mixture(styrene: t-butyl acrylate=0.45:0.05) were then added and thetemperature was raised to 50° C. in 10 min. Subsequently, the monomermixture was added (total amount: 17.0 g). After 3 h at 50° C. thereaction mixture was cooled down to room temperature.

EXAMPLE 7

[0052] 630 g (1.67 amino-mol equivalent) PAIM (56% solution in water)and 12% (0.2 mol) dodecyl acrylate (vinylog addition) were heated for 6h at 80° C. Subsequently, 326 g water was added whereafter the mixturewas cooled down to room temperature. Conversion of the acrylate was:>99%.

[0053] 1005 g of the above hydrophobised PAIM was then reacted with 155g (1.68 mol) epichlorohydrine (ECH) at 6° C. for 4 min. Subsequently,the temperature was increased until 20° C. was reached. The temperaturewas then increased until 50° C. and a viscosity of 120 mPa s wasreached. 287 ml water was then added and the temperature was raised to65° C. and kept at that temperature until the viscosity reached 100 mPas. The reaction was finalised by addition of 50 ml sulfuric acid (50%)and 513 ml water adjusting the pH to 3.5.

EXAMPLE 8

[0054] 309.5 g (0.81 amino-mol equivalent) PAIM (55% solution in water)and 15% (0.12 mol) benzyl chloride (alkylation reaction) were heated for6 h at 60° C. Subsequently, the mixture was cooled down to roomtemperature.

[0055] 125.5 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 6 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 9

[0056] 350 g (0.91 amino-mol equivalent) PAIM (55% solution in water)and 15% (0.14 mol) 2-ethylhexyl glycidyl ether (alkylation reaction)were heated for 7.5 h at 60° C. Subsequently, the mixture was cooleddown to room temperature.

[0057] 130.4 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 10

[0058] 274 g (0.71 amino-mol equivalent) PAIM (55% solution in water)and 3.8% (0.027 mol) alkyl ketene dimer (C18-chains) were heated for 6 hat 60° C. Subsequently, the mixture was cooled down to room temperature.

[0059] 127.2 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 11

[0060] 274 g (0.71 amino-mol equivalent) PAIM (55% solution in water)and 5% (0.036 mol) alkenyl succinic anhydride (C18-chains) were heatedfor 6 h at 60° C. Subsequently, the mixture was cooled down to roomtemperature.

[0061] 124.3 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 12

[0062] 185.4 g (0.48 amino-mol equivalent) PAIM (54% solution in water)and 10% (0.048 mol) hexanediol diacrylate (90%) were heated for 4.5 h at80° C. Subsequently, the mixture was cooled down to room temperature.Conversion of acrylate: >99%.

[0063] 124.0 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

Application Testing

[0064] Paper sheets were prepared on the dynamic sheet former“Formette”. The furnish consisted of 35% CTMP and 65% TCF refined to 25°SR. The paper was artificially cured 10 min at 105° C. beforeconditioning the paper according to DIN 5312. Tensile testing was doneas described in DIN 53112. For wet tensile testing the paper was soaked60 min at room temperature. For comparison reasons, data on paperprepared by using a conventional polyaminoamide epichlorohydrin resinhas also been given. It is to be noted that the paper sheets belowappearing in tables 1-4 have been tested at three different occasionsusing different addition levels of the wet strength agent used. Inexample 1-6, 20 kg wet strength agent was added/tonne cellulosic fibres.The grammage was 55 g/m². In examples 7-12, the grammage was 30 g/m² andthe addition levels of wet strength resin were 15, 20 and 30 kg/tonnecellulosic fibres. As a consequence thereof, observed values of relativestrength vary between each occasion. A reference resin, ie. aconventional resin, has therefore been measured at each occasion asappears from the tables 1-4 below. As can be seen from the examples, thewet strength resins and agents show superior effect in view of theconventional resin used as reference at the same addition levels. TABLE1 Dry tensile Wet tensile Relative wet Sample index in Nm/g index inNm/g strength in % Conventional resin 49 13 27 Example 1 37 14 37 WetStrength Emulsion Example 2 51 15 30 Wet Strength Emulsion Example 3 3712 32 Wet Strength Resin Example 3 37 13 34 Wet Strength EmulsionExample 4 33 12 36 Wet Strength Emulsion Example 5 35 11 31 Wet StrengthEmulsion Example 6 37 12 33 Wet Strength Emulsion

[0065] TABLE 2 Sample Dry tensile index Wet tensile index Relative wet15 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 40.39.7 23.9 Example 7 31.6 9.3 29.5 Wet strength resin Example 8 38.3 11.028.7 Wet strength resin Example 9 33.6 9.0 26.7 Wet strength resinExample 10 40.3 10.7 26.6 Wet strength resin Example 11 35.3 10.7 30.2Wet strength resin Example 12 38.6 10.3 26.7 Wet strength resin

[0066] TABLE 3 Sample Dry tensile index Wet tensile index Relative wet20 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 41.610.3 24.8 Example 7 31.6 9.3 29.5 Wet strength resin Example 8 38.0 10.828.5 Wet strength resin Example 9 35.0 10.0 28.6 Wet strength resinExample 10 39.3 11.0 28.0 Wet strength resin Example 11 35.0 11. 31.4Wet strength resin Example 12 37.3 10.7 28.6 Wet strength resin

[0067] TABLE 4 Sample Dry tensile index Wet tensile index Relative wet30 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 40.010.7 26.7 Example 7 31.6 10.0 31.6 Wet strength resin Example 8 39.311.7 29.7 Wet strength resin Example 9 34.0 11.0 32.4 Wet strength resinExample 10 38.3 11.3 29.6 Wet strength resin Example 11 34.3 11.3 33.0Wet strength resin

1. Method for preparing a wet strength agent comprising a first step ofreacting a nitrogen-containing polymer with a hydrophobic compound toform hydrophobic side-chain substituents on the polymer, a second stepof reacting the hydrophobised nitrogen-containing polymer obtained witha crosslinker to form a cationic nitrogen-containing resin, and a thirdstep comprising forming of particles by emulsion polymerisation of oneor more ethylenically unsaturated monomers in the presence of the wetstrength resin formed.
 2. Method according to claim 1 wherein thenitrogen-containing polymer is a polyamine or a polyaminoamide. 3.Method according to any of the claims 1-2 wherein the first step is avinylog addition or alkylation where the hydrophobic compound isselected from alkyl(meth)acrylates, alkyl(meth)acrylamides, alkylsulphonates, alkyl sulphates, diazo compounds, ethers, or epoxides ormixtures thereof.
 4. Method according to claim 1 wherein the hydrophobiccompounds comprise a hydrophobic chain having 6-40 carbon atoms. 5.Method according to claim 1 wherein the hydrophobic compounds comprise ahydrophobic chain having 8-40 carbon atoms.
 6. Method according to claim1 wherein the hydrophobic compound contains a chain of atoms containingat least one hetero atom.
 7. Method according to claim 1 wherein thecrosslinker is epichlorohydrin.
 8. Method according to claim 1 whereinthe monomers are selected from styrene, butadiene, alkyl(meth)acrylates, alkyl(meth)amides, (meth)acrylonitrile, vinyl acetate,or vinyl amide, or mixtures or derivatives thereof.
 9. Method as claimedin claim 1 wherein the hydrophobic compound is a saturated compound, oran unsaturated compound, resulting in a nitrogen-containing polymerhaving saturated side-chain substituents.
 10. Paper wet strength agentobtainable by a method as defined in claim
 9. 11. Paper wet strengthagent comprising a wet strength resin comprising cationicnitrogen-containing polymers having hydrophobic saturated side-chainsubstituents and groups derived from a crosslinker; and polymericparticles.
 12. Paper wet strength agent according to claim 11 whereinthe hydrophobic side-chain substituents contain a hydrophobic groupattached to a nitrogen atom of the nitrogen-containing polymer via achain of atoms comprising 6-40 carbon atoms.
 13. Paper wet strengthagent according to any of claims 11-12 wherein the hydrophobicside-chain substituents are selected from derivatives ofalkyl(meth)acrylates, alkyl(meth)acrylamides, alkyl sulphonates, alkylsulphates, diazo compounds, ethers, or epoxides or mixtures thereof. 14.Method for preparing a wet strength resin comprising a first step ofreacting a nitrogen-containing polymer with a hydrophobic compound toform hydrophobic side-chain substituents, in which said hydrophobiccompound is selected from alkyl(meth)acrylates, alkyl(meth)acrylamides,alkyl sulphonates, alkyl sulphates, diazo compounds, ethers, or epoxidesor mixtures thereof, and a second step of reacting the hydrophobisednitrogen-containing polymer obtained with a crosslinker to form acationic nitrogen-containing resin.
 15. Method according to claim 14wherein the hydrophobic compound contains 6-40 carbon atoms.
 16. Methodas claimed in claims 14 or 15 wherein the hydrophobic compound is asaturated compound, or an unsaturated compound, resulting in anitrogen-containing polymer having saturated side-chain substituents.17. Paper wet strength resin obtainable by a method as defined in claims14-15.
 18. Paper wet strength resin comprising cationicnitrogen-containing polymers having saturated hydrophobic side-chainsubstituents selected from compounds derived from alkyl(meth)acrylates,alkyl(meth)acrylamides, alkyl sulphonates, alkyl sulphates, diazocompounds, ethers, or epoxides or mixtures thereof; and groups derivedfrom a crosslinker.
 19. Process for production of tissue papercomprising addition of a paper wet strength resin or agent, comprising acationic nitrogen-containing polymer having hydrophobic side-chainsubstituents, to an aqueous cellulosic suspension.
 20. Process accordingto claim 19, wherein the paper wet strength resin or agent is added inan amount of from about 5 to about 50 kg/tonne dry cellulosic fibres.21. Process according to claims 19 or 20, wherein the paper wet strengthresin is added in an amount of from about 15 to about 50 kg/tonne drycellulosic fibres.
 22. Process according to claim 20, wherein the paperwet strength resin is added in an amount of from about 25 to about 50kg/tonne dry cellulosic fibres.
 23. Process according to claim 20,wherein a dry strength agent is added in combination with the paper wetstrength resin or agent.
 24. Process according to claim 20, wherein theproduced tissue paper has a grammage lower than about 70 g/m². 25.Tissue paper comprising a paper wet strength resin or agent comprising acationic nitrogen-containing polymer having hydrophobic side-chainsubstituents.
 26. Tissue paper according to claim 25, wherein the tissuepaper comprises a paper wet strength resin or agent in an amount fromabout 5 to about 50 kg/tonne dry cellulosic fibres.
 27. Tissue papercomprising a paper wet strength resin or agent obtainable by a methodaccording to any of claims 19-24.